Recently, recycling technology has been developed in various fields from the viewpoint of building “resource recycling-oriented society”. In the fields related to woody material, such as thinned wood, waste wood remaining in forest land, scrap wood generated in wood processing industry (e.g. wood powder and listings) and building demolition materials, a technique has been drawn attention which provides a woody material with hot-press moldability that cellulose forming woody material does not inherently have, in order to promote use of these waste biomass. It is expected that, if a woody material is provided with thermal or pressure plasticity (hot-press fluidity), the woody material has plastic processability of processing/molding in desired shapes, so that new application fields will be developed.
As a technique for imparting hot-press moldability to a woody material, the present inventors have developed a technique in which cellulose diacetate and an acetylated woody material are oligoesterified (for example, see M. Yoshioka “Biodegradable Plastics from Cellulose and Lignocellulosics”, “Biopolymers Vol. 9”, Steinbuhel A. & Matsumura S. (eds.), p. 201-235, Wiley-VCH, Weinheim, Germany, 2003; M. Yoshioka and N. Shiraishi: Biodegradable Plastics from Cellulose, Mol. Cryst. and Liq. Cryst., 353, p. 59-73 (2000)).
However, the conventional techniques for imparting hot-press moldability to the woody material have problems in that profitability in industrial scale manufacture is poor, since cellulose diacetate, which is an expensive material, is used and a product becomes expensive.
On the other hand, a conventional technique in which cellulose and a woody material are oligoesterified for plasticizing can use cheap biomass materials, such as wood powder, and can manufacture industrial products in a small scale on an economically viable basis.
As for a technique to oligoesterify the woody material, there can be mentioned, for example, an invention disclosed in Japanese unexamined patent publication Kokai No. 60-83806 entitled “Method for producing modified wood chips”. According to this conventional technique, dibasic acid anhydride and monoepoxy compound are subjected to addition-esterification with wood chips to thereby obtain modified wood chips, and the resultant modified wood chips are subjected to hot-press processing, to thereby obtain a khaki-colored sheet or board having a hardened surface with excellent thermal resistance and water resistance. However in this conventional technique, it is necessary to add a cross-linking agent which functions also as a plasticizer, and to conduct hot-press processing at a high temperature of 180° C.-200° C. or more under a high pressure of 300 kg/cm2 or more, in order to form modified wood chips. The molding temperature that exceeds 180° C. is not preferred, since it initiates thermal decomposition of the woody material, resulting in poor quality of the product and poor energy efficiency in production.
Japanese examined patent publication kokoku No. 1-58208 entitled “method for producing plastic-like woody molded product” discloses the following technique. First, woody material is subjected to addition-esterification with polybasic acid anhydride and monoepoxy compound having unsaturated double bond, to thereby obtain an oligoesterified cellulose (a). An oligomer (b) is also obtained from polybasic acid anhydride and monoepoxy compound having unsaturated double bond. To a composition including the oligoesterified cellulose (a) and the oligomer (b) obtained as such is added a small amount of a radical polymerization initiator, if necessary, and the composition is subjected to hot-press molding where plasticization and cross-linking by polymerization of double bond are performed at the same time to thereby obtain a molded product. The thus-obtained plastic-like woody molded product is excellent in physical properties, such as dimensional stability, mechanical property and thermal deformation temperature.
However, in the plastic-like woody molded product according to this conventional technique, cross-linking is conducted by polymerization of double bond during molding. Therefore, after this molding, if hot-press molding is repeated again, the product cannot be plasticized any more, and thus cannot be recycled.
Moreover, Japanese Patent No. 2661779 entitled “thermoplastic woody composition for molding and method for producing the same” discloses a technique in which, when a woody material component is subjected to addition-esterification with polybasic acid anhydride and monoepoxy compound, a composition for molding is obtained by terminating reaction while unreacted polybasic acid anhydride and/or monoepoxy compound remain. According to this technique, there is provided a feature in which even a molded product that has once undergone hot-press molding can be recycled through hot-press molding, when the molded product is powdered and polybasic acid anhydride and/or monoepoxy compound are added thereto.
However, in the thermoplastic woody composition for molding according to this conventional technique, hot-press molding conditions (temperature, pressure and the like) for obtaining a desired hot-press fluidity (thermoplasticity) does not reach sufficiently low temperature and pressure regions.
For plasticizing techniques to improve this hot-press fluidity, there have been widely known a method in which a low-molecular plasticizer is used, a method in which a high-molecular plasticizer is used, a method of intramolecular plasticization, such as introduction of substituent, and the like. Specifically, it has been known that, as a modified method using a low-molecular plasticizer, use of bisphenol A diglycidyl methacrylate as a plasticizer improves a fluidity and thus moldability. However, some plasticizers are suspected as endocrine-disrupting chemicals (what is called “environmental hormone”) and thus it is not desirable to use such a plasticizer.
In the conventional techniques mentioned above, the reason for a poor hot-press fluidity (thermoplasticity) of oligoesterified woody material (cellulosic material) is believed to be as follows. In the conventional oligoesterification, polymerization does not reach a core of a cellulose crystal in a starting material, so that unreacted portions remain as a tangled form. In other words, since the oligoester substituents are locally densely present in the woody material or an amorphous portion of cellulose, a satisfactory hot-press fluidity (thermoplasticity) cannot be obtained in the conventional thermoplastic cellulose composition.